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Post-marketing observational studies permit the evaluation of drug safety in a large number of patients in a real-world setting, where practice patterns, including off-label use, can be assessed. Such studies have limited ability to determine causation, but they can detect signals that may suggest a safety concern.
In an observational study, decisions to use specific drugs are made by local physicians, according to their perceptions of the risks and benefits for particular patients. Such treatment allocation results in an imbalance in demographic and risk factors between patients given the drug of interest and those given an alternative or no treatment. An imbalance in various clinical factors will directly influence safety outcomes when patients at higher risk receive the drug. Therefore, appropriate statistical methods must be used to adjust for the nonrandom assignment to treatment groups.1
One way to address imbalances between groups is to use a propensity score that incorporates confounders and other covariates into a model predicting the probability of assignment to a particular treatment. This score can be used for adjustment or for matching patients who have similar probabilities of receiving a treatment. Differences in outcomes between treated and untreated patients (or patients treated with a comparison drug) with equal propensity scores provide a less biased estimate of treatment effect.
Aprotinin was approved by the FDA in 1993 as a means of reducing perioperative blood loss and the need for blood transfusion in patients undergoing coronary-artery bypass grafting. Neither the clinical trial database that led to approval nor the numerous randomized, controlled trials conducted after approval identified an association between aprotinin and any short-term increase in the risk of death or nonfatal cardiovascular events or any serious renal toxic effects (except for a transient increase in the serum creatinine concentration). However, in early 2006, two observational studies were published that raised serious concerns about the drug's safety.2,3
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The second study, by Karkouti et al.,3 compared the risks associated with aprotinin with those associated with tranexamic acid in 898 patients undergoing high-risk cardiac surgery. The authors used a propensity score to match patients who had been given the different treatments — an approach that permitted the identification of a population of patients who were subsequently fully matched according to a large number of covariates. Calculations from the study data, performed by the Colorado Prevention Center, indicated no significant difference between the two treatments in the overall risk of myocardial infarction (odds ratio, 1.20; 95% CI, 0.52 to 2.75), stroke (odds ratio, 1.15; 95% CI, 0.56 to 2.40), or death (odds ratio, 0.91; 95% CI, 0.56 to 1.46). However, among patients who had abnormal renal function at baseline, those given aprotinin had a significantly increased risk of postoperative renal dysfunction (odds ratio, 1.69; 95% CI, 1.07 to 2.69). Because this study included fewer patients than the study by Mangano et al., it had less power to detect a safety problem.
These observational studies did not permit a definitive conclusion about cardiovascular or renal risk, but they did raise concern. The FDA appropriately issued a public health advisory about the potential risks posed by aprotinin and urged physicians to monitor their patients carefully for renal, cardiac, and cerebral toxic effects.4 The agency also convened a meeting of the Cardiovascular and Renal Drugs Advisory Committee on September 21, 2006, to evaluate the evidence on the cardiovascular and renal toxic effects of aprotinin. Our committee reviewed the published studies and the global safety and efficacy database submitted by Bayer, which included 45 randomized, controlled trials involving 2249 patients who received aprotinin and 2164 who received placebo.
The Bayer safety analysis was confined to the unadjudicated adverse events reported within 7 days after the administration of aprotinin, which included 520 deaths or serious cardiovascular or renal events. The results did not reveal any increased risk of fatal or nonfatal cardiovascular events. According to the Colorado Prevention Center's calculations based on Bayer's data, the odds ratio for death was 1.14 (95% CI, 0.80 to 1.62); for myocardial infarction, 1.17 (95% CI, 0.93 to 1.49); for stroke, 0.71 (95% CI, 0.42 to 1.18); and for renal failure, 1.15 (95% CI, 0.74 to 1.78). Two studies in which myocardial infarctions were adjudicated by an independent review committee showed a nonsignificant increase in the risk of myocardial infarction, with a point estimate of the odds ratio as high as 2.24 (95% CI, 0.56 to 9.00).
How can the results of these trials and the study by Karkouti et al. appear to be so different from the findings of Mangano et al.? Part of the problem may derive from the inability of observational studies to identify and measure all relevant covariates that may influence the outcome; similar studies using a few different covariates can sometimes come to opposite conclusions.
FDA advisory committee meetings usually include an independent review of data — either those provided by the sponsor or those used by the authors of relevant published studies. This independent and transparent process becomes even more critical in the evaluation of observational studies, whose conclusions can be highly influenced by the statistical methods used and the inability to identify all confounding factors. Unfortunately, Mangano did not give the FDA or the committee full access to his data, which would have allowed the agency to perform an independent review and analysis to validate his group's findings. Although there are many legitimate concerns with regard to the sharing of data (for instance, confidentiality and the need for informed consent), the lack of independent review greatly limited the committee's ability to draw conclusions from the study. Having reviewed all the data available, the committee decided that there was insufficient evidence to require an additional warning on aprotinin's labeling and agreed that the clinical data supported an acceptable safety and efficacy profile for aprotinin.
Days after the committee meeting, the FDA was made aware of additional observational data from the sponsor that had not been presented at the meeting. Bayer evidently had commissioned an observational study involving 67,000 patients who were given aprotinin.5 According to the initial FDA review of data from that study, aprotinin may be associated with "increased risk for death, kidney failure, congestive heart failure and stroke." The failure of Bayer to disclose all its data on aprotinin seriously undermined the advisory committee process and hindered the safety review.
Although observational studies cannot be definitive (and so should evoke measured responses), they can provide important new safety information that can direct further scientific and regulatory actions — if their findings can be confirmed. Since further analysis of the new data provided by Bayer is ongoing at the FDA, conclusions about the overall safety of aprotinin cannot be drawn at this time. Still, the aprotinin story demonstrates that full disclosure and a transparent process are essential in evaluating the findings of all studies germane to drug safety and the public health.
Source Information
Dr. Hiatt is a professor of medicine at the University of Colorado School of Medicine and president of the Colorado Prevention Center — both in Denver — and is the current chair of the Cardiovascular and Renal Drugs Advisory Committee of the FDA. Opinions expressed in this article are those of the author and do not necessarily reflect the opinions of the FDA or the advisory committee.
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Related Letters:
Judging the Safety of Aprotinin
Mangano D. T., Rieves R. D., Weiss K. D.
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N Engl J Med 2006;
355:2261-2262, Nov 23, 2006.
Correspondence
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